The particle filters which are used increasingly for cleaning the exhaust gas in diesel engines and which, apart from filtering out other harmful gas constituents of the engine exhaust gas, serve in particular for filtering out soot particles contained in the exhaust gas, must be subjected to frequent cleaning (regeneration) for retaining their operability. In this respect especially the thermal regeneration of particle filters has turned out to be effective, in which the soot particles contained in the particle filter are ignited and burnt by the introduction of high-temperature heating gases (of approx. 600.degree. C. to 900.degree. C.).
For carrying out a thermal regeneration of particle filters, there are in essence three methods known, namely stationary regeneration, alternating regeneration and full flow regeneration. In case of a stationary regeneration, burning out of the particle filter takes place, while the vehicle engine is at a standstill by means of a heating means provided for this purpose and being independent of the engine. Alternating regeneration renders possible a thermal regeneration of the particle filter while the vehicle is in operation. To this end, two particle filters are connected in parallel, and in alternating manner one particle filter has engine gas flowing therethrough while a heating gas heated by the engine-independent heating means flows through the other particle filter, which is disconnected from the exhaust gas system, during the time of thermal regeneration. With full flow regeneration, in which the regeneration also takes place during operation of the vehicle, the particle filter is disposed permanently in the exhaust gas flow which is subjected during the time of regeneration to a heating gas flow which is produced by the engine-independent heating means and mixed with the engine exhaust gas and together with the latter is introduced into the particle filter for obtaining the afore-mentioned gas temperature necessary for regeneration.
Irrespective of the choice of the regeneration method performed, a thermal regeneration of course has to be carried out only when a certain load condition of the particle filter has been reached, in which either the filter has lost its effectiveness or in which the exhaust gas back pressure produced by the clogged filter has a disadvantageous effect on the engine power, or in which the filter, by further loading thereof, would be thermally destroyed during the next regeneration due to the heat set free in the combustion of soot.
As practical possibilities of continuously monitoring the load condition of a particle filter during operation are not yet known so far, certain fixed operational intervals are set by the manufacturers of particle filters, defining when a thermal regeneration is to be carried out. As the actual load condition is not known, the regeneration intervals are selected such that also under extreme operating conditions of the engine, such as frequent short-distance operation with extreme soot formation in the engine exhaust gases, there is provided sufficient security in the intervals for guaranteeing that the regeneration definitely can take place in due time before occurrence of the afore-described harmful effects. The regeneration of a particle filter in the case of operation of the engine with low soot formation thus necessarily takes place at a time at which such regeneration actually would not be necessary.
One has started to adapt the regeneration intervals in consideration of the predominantly prevailing operating conditions, such as e.g. short-distance or long-distance operation, the particular conditions of use of a particle filter, but even this refined pattern of the regeneration intervals turns out to be too coarse in practical application.
The generally known relationship between the pressure drop in a flow medium when flowing through a filter or the pressure increase in front of the filter, respectively, and the degree of clogging or loading of the filter, which as such holds only for filters having a constant flow therethrough, cannot be used alone for determining the load condition of a particle filter because of the predominantly varying operation of a combustion engine. Rather, for example the volume flow through the filter must be taken into consideration, which changes depending on the engine speed.